Field of Art
The present disclosure relates to an information processing apparatus, an image reconstruction method, and a computer-readable medium, in particular ones suited for use in creation of a reconstructed image by a sequential image reconstruction method.
Description of the Related Art
For example, an X-ray computed tomography (CT) apparatus irradiates a target with X rays from a plurality of directions to create a plurality of X-ray projection images, and computes an X-ray-absorption coefficient inside the target from the plurality of X-ray projection images, and makes an image of their distribution. As an image reconstruction method, there is a method called sequential image reconstruction (or sequential approximate image reconstruction or the like), for example.
By the sequential image reconstruction method, first, an area where an image is to be reconstructed is divided into a plurality of voxels, and an initial image (initial values of the voxels (absorption coefficients)) is given and forward projection is performed. The ratio of a projection image obtained by the forward projection and a projection image detected by an X-ray detection panel through actual projection of X rays is subjected to back projection, and the values of voxels are repeatedly updated until a predetermined condition for completion of computation is satisfied. Japanese Patent No. 5133690 discloses a technique for back projection by such the sequential image reconstruction by which the projection image is interpolated based on the positions of the voxels and back projection is performed by the use of the interpolated projection image.
To obtain an image with few artifacts by the sequential image reconstruction method, it is important that a matrix indicative of forward projection and a matrix indicative of back projection are in a relationship of transposed matrix. It would be easy to satisfy this property when respective matrix elements can be stored in a storage device inside the apparatus. However, when the required size of a reconstructed image is large, the number of the matrix elements grows immensely, and storing those matrix elements in the storage device is not realistic in many cases.
In the computation on forward projection and the computation on back projection, it is not easy to create matrix elements as necessary and, at the same time, execute efficient parallel computation when the matrixes of forward projection and back projection are in the relationship of transposed matrix. This is because, when a computation method is set up such that either forward projection or back projection is suited to parallel computation, the efficiency of parallel computation is generally sacrificed in the other computation in which the other projection acts as a symmetric matrix. For example, when the computation for back projection corresponding to the transposed matrix in the ray-driven computation for forward projection is parallelized, atomic operation sacrificing parallelism is included.
Accordingly, in the reconstruction of a large-sized image, the computation on forward projection and the computation on back projection are executed in parallel by algorithms convenient to their parallel computation, and the relationship of transposed matrix between forward projection and back projection is abandoned in many cases.